Video signal processing in a conventional television camera will be described below.
(Television Camera: FIG. 7)
The configuration of the television camera will be explained with reference to FIG. 7. FIG. 7 is a block diagram showing the configuration of the television camera.
As shown in FIG. 7, the television camera 1 includes a lens 2, a prism 3, an imaging element 4, a CDS (Correlated Double Sampling) circuit 5, a VGA (Variable Gain Amplifier) circuit 6, an A/D (Analog to Digital) converter 7, a video signal processing unit 8, a video signal output unit 9, a TG (Timing Generator) 10, and a CPU (Central Processing Unit) 11.
Next, the operation of the television camera will be explained.
First, a subject image passes through the lens 2 of the television camera 1. The subject image is divided into three colors of R (red), G (green) and B (blue) in the prism 3 and is converted into electrical signals of R, G and B in the imaging elements 4R, 4G and 4B.
Thereafter, the electric signals pass through the CDS circuits 5R, 5G and 5B and are amplified by the VGA circuits 6R, 6G and 6B. Then, the electric signals are converted into digital signals from analog signals in the A/D converters 7R, 7G and 7B. After the electric signals are subjected to various signal processes in the video signal processing unit 8, a television signal is outputted from the video signal output unit 9.
The TG 10 generates a timing signal for driving the imaging element 4 and the CDS circuit 5, and the CPU 11 as a system controller controls the circuits of the respective units.
(Conventional First Contour Enhancement Processing Circuit: FIG. 8)
A conventional first contour enhancement processing circuit as a contour enhancement processing circuit in the video signal processing unit 8 will be described with reference to FIG. 8. FIG. 8 is a block diagram showing the configuration of a conventional first contour enhancement processing circuit.
The conventional first contour enhancement processing circuit is provided in the video signal processing unit 8. As shown in FIG. 8, the conventional first contour enhancement processing circuit includes a delay circuit 12, an adder 13, a contour extraction circuit 14 and a gain circuit 15.
In the operation of the conventional first contour enhancement processing circuit, a video signal is inputted to the delay circuit 12 and the contour extraction circuit 14. The delay circuit 12 delays the signal processing time of the contour extraction circuit 14 and the gain circuit 15, and outputs a delayed video signal 12a to the adder 13.
The contour extraction circuit 14 extracts a contour component from the inputted video signal, and outputs a contour signal 14a to the gain circuit 15.
The gain circuit 15 amplifies the contour signal 14a according to the amplification amount (gain) inputted from the CPU 11, and outputs a contour signal 15a to the adder 13. The adder 13 adds the contour signal 15a and the delayed video signal 12a whose phase is brought into coincidence with that of the contour signal 15a by the delay circuit 12, thereby outputting a contour-enhanced video signal.
(Image Example: FIGS. 9A and 9B)
Next, an image example related to contour enhancement will be described with reference to FIGS. 9A and 9B. FIG. 9A is a view showing an original image example (an image example before contour enhancement), and FIG. 9B is a view showing an image example after contour enhancement.
While FIG. 9A shows an example of an original image (an image before contour enhancement), FIG. 9B shows an example of a contour-enhanced image (an image after contour enhancement). In FIG. 9B, an image signal is processed so that the contour portion of an image pattern becomes clear.
(Signal Waveform Showing Ideal Contour Enhancement Process: FIGS. 10A to 10D)
A signal waveform showing a noise-free ideal contour enhancement process will be described with reference to FIGS. 10A to 10D. FIGS. 10A to 10D are signal waveform diagrams showing an ideal contour enhancement process. In FIGS. 10A to 10D, 11A to 11D and 14A to 14E, the vertical axis represents a video signal level, and the horizontal axis represents a video signal waveform.
FIG. 10A shows a noise-free ideal signal waveform in an image example shown in FIG. 9A. In FIG. 9A, the image is dark on the left side and is gradually brightened toward the right side. Thus, the video signal level is increasing stepwise in the horizontal axis direction.
In FIG. 10B, there is shown a contour signal waveform obtained by extracting a contour from the noise-free ideal video signal using the contour extraction circuit 14.
In FIG. 10C, there is shown a signal waveform obtained by amplifying the contour signal 14a using the gain circuit 15. In FIG. 10D, there is shown a contour-enhanced video signal waveform obtained by adding the amplified contour signal to the delayed video signal using the adder 13. The contour-enhanced video signal is the image example shown in FIG. 9B as an image example.
Meanwhile, a noise is contained in an actual video signal. Thus, a noise is also contained in the contour signal 14a extracted by the contour extraction circuit 14. As the contour signal 14a is amplified by the gain circuit 15, the noise is also amplified. When the contour enhancement is strengthened, the noise of the video signal and the contour-enhanced noise are synthesized in the outputted contour-enhanced video signal.
(Signal Waveform when Noise is Contained in Video Signal: FIGS. 11A to 11D)
Next, a signal waveform when a noise is contained in the video signal will be described with reference to FIGS. 11A to 11D. FIGS. 11A to 11D shows a signal waveform when a noise is contained in the video signal.
FIG. 11A shows an example in which a noise is contained in the video signal and shows a video signal waveform containing a noise having a characteristic that the noise increases in proportion to a video signal level.
In FIG. 11B, there is shown a contour signal waveform obtained by extracting a contour from a noise-containing video signal using the contour extraction circuit 14. Since a contour of the noise is also extracted, a noise is also contained in the contour signal waveform.
In FIG. 11C, there is shown a contour signal waveform obtained by amplifying the contour signal using the gain circuit 15. Thus, the noise contained in the contour signal is also amplified.
In FIG. 11D, there is shown a signal waveform of the contour-enhanced video signal obtained by adding (synthesizing) the amplified contour signal to the delayed video signal using the adder 13. A noise is contained in the delayed video signal. A noise is also contained in the contour signal waveform and is amplified. Therefore, the contour is enhanced in the contour-enhanced video signal obtained by synthesizing the delayed video signal and the contour signal waveform. Since the noises are overlapped and increased, the image has a rough feeling.
(Conventional Second Contour Enhancement Processing Circuit: FIG. 12)
In order to prevent the above noise, it is usual to perform a crisp process after the contour extraction circuit 14. The crisp process is a process of removing a noise signal from the contour signal.
Next, as a contour enhancement processing circuit for performing the crisp process, a conventional second contour enhancement processing circuit will be described with reference to FIG. 12. FIG. 12 is a block diagram showing the configuration of the conventional second contour enhancement processing circuit.
As shown in FIG. 12, the conventional second contour enhancement processing circuit has a configuration in which a crisp circuit 16 is provided between the contour extraction circuit 14 and the gain circuit 15 in the contour enhancement processing circuit of FIG. 8.
The delay circuit 12 outputs a delayed video signal 12b obtained by delaying the inputted video signal by the time of the signal processing in the contour extraction circuit 14, the crisp circuit 16 and the gain circuit 15.
The crisp circuit 16 removes a noise from the contour signal 14a of the contour component extracted by the contour extraction circuit 14 using a noise reduction amount (crisp) signal and outputs a contour signal 16a. 
The gain circuit 15 amplifies the noise-removed contour signal 16a by the amplification amount (gain) inputted from the CPU 11 and outputs the amplified contour signal 15b to the adder 13.
The adder 13 adds the contour signal 15b to the video signal 12b whose phase is matched with that of the contour signal 15b by the delay circuit 12, thereby obtaining a contour-enhanced video signal.
The specific operation of the conventional second contour enhancement processing circuit will be described.
The conventional second contour enhancement processing circuit obtains a contour signal 16a, from which a noise component is removed by the crisp circuit 16, from the contour signal 14a containing a noise. If the contour signal 16a is amplified by the gain circuit 15, a contour signal 15b in which only a contour portion is enhanced is obtained. A clear image with little noise is obtained by adding the contour-enhanced contour signal 15b and the video signal 12b using the adder 13. In this regard, a crisp signal is a signal transmitted from the CPU 11 that designates a noise removal amount (crisp amount) of the crisp circuit 16.
(Signal Waveform Indicating Crisp Function: FIG. 13)
A crisp function will be described with reference to FIG. 13 is a signal waveform diagram showing the crisp function.
As shown in FIG. 13, the noise of the contour signal containing a noise is removed by the crisp circuit 16. However, the contour signal is also reduced by the amount of noise reduction. Thus, the gain circuit 15 performs gain amplification by amplifying the contour signal to have the original signal level. The process of removing a noise by the crisp function is referred to as a crisp process.
(Signal Waveform Showing Conventional Second Contour Enhancement Process: FIGS. 14A to 14E)
A signal waveform showing a conventional second contour enhancement process will be described with reference to FIGS. 14A to 14E. FIG. 14A to 14E are signal waveform diagrams showing the conventional second contour enhancement process.
In FIG. 14A, there is shown an example of a video signal waveform in which a noise component is contained in a video signal. The noise signal having a noise component has a characteristic that the noise increases in proportion to the video level.
In FIG. 14B, the contour is extracted from the video signal by the contour extraction circuit 14, and the contour of a noise is also extracted. Therefore, a noise component is also contained in the contour signal waveform.
In FIG. 14C, there is shown a signal waveform from which a noise is reduced by the crisp circuit 16. The crisp amount in the crisp process is constant irrespective of the video level.
In FIG. 14D, there is shown a contour signal waveform obtained by amplifying the contour signal in the gain circuit 15. In this regard, only the contour is amplified in the portion where the noise component is removed in the crisp process. However, the noise component is also amplified in the remaining portion.
In FIG. 14E, there is shown a video signal waveform in which the contour signal is added to the video signal by the adder 13. In the contour-enhanced video signal, the contour is enhanced. However, the remaining noise component is also increased. Therefore, the image has a rough feeling. That is to say, the contour-enhanced video signal shown in FIG. 14E is obtained by synthesizing the video signal waveform of FIG. 14A and the contour signal waveform shown of FIG. 14D.
(Contour Signal Waveform for Explaining Problem of Prior Art: FIG. 15)
Next, a contour signal waveform for explaining the problem of prior art will be described with reference to (a) to (c) of FIG. 15. (a) to (c) of FIG. 15 are contour signal waveform diagrams for explaining the problem of prior art.
In (a) of FIG. 15, there is shown an example in which the contour signal contains a noise signal having a characteristic that the noise component increases in proportion to the video level of the video signal.
Since the crisp amount is constant when performing the crisp process, the influence on the contour signal waveform varies depending on the degree of the crisp amount.
In (b) of FIG. 15, there is shown a case where the crisp amount is matched with the noise component of the portion where the video level of the video signal is high. The noise component is completely eliminated, but the contour signal is small. When the contour signal is originally small, the contour may disappear.
In (c) of FIG. 15, there is shown a case where the crisp amount is matched with the noise component of the portion where the video level of the video signal is low. The noise component having a high video level of the video signal is not removed and remains.